A research team at Pennsylvania State University in the USA evaluated residential heat pumps paired with thermochemical energy storage (TCES) for long-duration energy storage. The researchers aimed to determine whether storing heat ahead of cold spells could reduce heating-related electricity demand when heat pumps become less efficient and the grid faces greater pressure.
“In this paper, we model a TCES system using potassium carbonate (K₂CO₃) as the thermochemical material (TCM),” the group explained. “This system can be thermally charged by an electric heat pump well in advance and discharged to meet the building’s heating load during the heating season. The optimal operation is assessed during the most stressful time of the heating season, based on projections for 2050.”
The researchers used simulation and linear optimization to evaluate an air-source heat pump coupled with potassium carbonate-based TCES and a battery in a 220.82 m² single-family home under New York City conditions. Hourly grid projections for a 95% decarbonized electricity system in 2050 were translated into a varying building electricity-demand limit.
The TCES model represented heat storage through salt dehydration and heat release through hydration, with approximately 80 C water required for charging and 35 C water supplied to radiant heating during discharge. The team further optimized hourly operation to minimize electricity consumption while meeting heating demand and respecting the electricity limit.
Reference charging/discharging efficiencies were 45%/80% for TCES and 95%/95% for the battery. Across 96 simulations, they varied heat-pump performance, TCES specific heating power, and charging efficiency to assess their effects on required storage capacities.
According to the results, the reference system required 68 kWh of thermal storage and 168 kWh of battery capacity, with batteries providing most of the storage response. Higher warm-weather heat-pump coefficient of performance (COP), improved TCES charging efficiency, and greater specific heating power favored thermal storage, while better cold-weather COP reduced both storage requirements. In the scenario most favorable to thermal storage, the optimized TCES capacity requirement was 894 kWh.
“These findings suggest two potential pathways for future researchers and manufacturers to enable efficient, long-duration energy storage responses from heat pumps and TCES systems,” the academics noted. “The first approach involves increasing TCES’s charge efficiency from 45% to 65%, while the second involves enhancing the heat pump’s COP by at least 25%, even if it degrades in cold climates.”
The researchers said further development of the proposed configuration could focus on enhancing the thermal storage system’s specific heating power. Such advances could enable TCES to provide a greater share of heating demand during periods of grid stress, reducing reliance on battery storage. However, the viability of the combined system would depend on improvements in component performance and the ability to integrate thermal storage with residential heating systems.
The article “Long duration energy storage through residential heat pumps with thermochemical energy storage” has appeared in Energy.
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